Hi @mattiaguariglia02,
this is because the property (prices or availability) was never requested from the API before. When you requested this for the first time the API prioritized that property (updating prices and availability at least every day) and updated prices. So now prices have been updated. Keep in mind that a property stays prioritized for 24 hours after the last API call. If no API call for that property is done in the last 24 hours it comes back to a standard priority.

For days that are not available, the default Airbnb property price (€350) is returned because Airbnb gives you prices only for available nights and there is no way to get the actual price for a booked night (as on the Airbnb site). So for nights that were already booked after your first API call, you will get €350 again. So you should compare returned prices with property availability. Or you can consider nights priced at €350 as booked.

As per the documentation, the price endpoints give you the medium price for the longer available date intervals we find at that moment (and it can be slightly different if you want prices for a different date interval). To get a precise quote for a specific interval you can use the real-time prices endpoint.